A steel retainer is used to retain needle rollers of many of radial type needle roller bearings, in which the needle rollers are arranged along a cylindrical surface having a central axis, and thrust type needle roller bearings, in which the needle rollers are arranged around a central axis so as to extend in radial directions. Some of such needle roller bearings, of which the needle rollers have a small diameter, include only the retainer besides the rolling elements, and others further include at least one bearing ring selected from inner and outer bearing rings. Because needle roller bearings of either of the above types are thin compared to rolling bearings including other rolling elements, but are still sufficiently large in load capacity, by using such bearings, it is possible to reduce the size of devices and machines such as industrial machines and motor vehicles in which such needle roller bearings are used. Thus, needle roller bearings are now increasingly taking the place of e.g. slide bearings.
According to how they are manufactured, steel retainers used in radial type needle roller bearings are roughly classified into ones formed by cutting pipe members, ones formed by blanking a plate member to obtain a disk-shaped blank, and pressing, i.e. drawing the disk-shaped blank, and ones formed by cutting a strip of plate to a required length, annularly bending the thus cut strip and welding its ends together. In order to improve productivity, in many cases, pockets are punched, a step is formed by pressing, and bent portions are formed while the retainer is in the state of the strip. On the other hand, steel retainers of thrust type needle roller bearings are formed by blanking or cutting a plate member and optionally pressing.
These steel retainers come in various shapes according to their intended use. But they all have a plurality of pockets for receiving needle rollers which are formed by punching and define bridges therebetween. The needle rollers, which are received in the respective pockets and roll therein, are brought into sliding contact with the side walls of the bridges. Some needle rollers may skew while rolling. If they skew, the needle rollers locally come into sliding contact with the side walls of the bridges, which results in extremely increased slide contact pressure therebetween. If such a needle roller bearing includes a bearing ring, the radially inner or outer surface of the retainer may be brought into sliding contact with the bearing ring. Thus, the sliding contact surface of such a retainer may suffer surface damage such as seizure besides wear.
Conventional steel retainers of conventional needle roller bearings are typically formed from low-carbon steel for machine structural use such as S15C, low-carbon alloy steel such as SCM415, and low-carbon cold drawn steel plate such as SPC which all contain not more than 0.15% by mass of carbon, for ease of cutting and pressing during manufacturing steps. In order to prevent wear of and surface damage to retainers made of such steels, i.e. steels containing not more than 0.15% by mass of carbon, after machining, such conventional retainers are subjected to surface heat treatment such as carburizing, carbonitriding or nitrocarburizing (as disclosed in Patent document 1). Otherwise, portions of their pockets that are brought into contact with the rolling elements are subjected to surface hardening such as induction hardening or shock hardening after carbonitriding as preheat treatment or without such preheat treatment (as disclosed in Patent document 2). In Patent document 2, after the surface hardening, the surface hardness increases to not less than 250 HV with the core hardness maintained at not more than 190 HV. It is also known to modify the surfaces of steel retainers to increase their wear resistance and surface fatigue resistance by subjecting the retainers to surface chemical treatment such as plating with a metal such as silver or copper, oxide film treatment such as manganese phosphate treatment (as disclosed in Patent document 3).
Patent document 1: JP patent publication 10-46318A (pages 3 and 4)
Patent document 2: JP patent publication 2000-205274A (pages 2 and 3)
Patent document 3: JP patent publication 11-303875A (pages 2 and 3)